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Biology LibreTexts

6.6: Genetic Code

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  • Can You Code?

    If someone asks you whether you can code, you probably assume they are referring to computer code. The image above represents an important code that you use all the time but not with a computer. It's the genetic code, and it is used by your cells to store information and make proteins.


    Figure 1

    What Is the Genetic Code?

    The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal. There are 20 common amino acids in proteins. With four bases forming three-base codons, there are 64 possible codons. 61 codons are more than enough to code for the 20 amino acids, thus more than one codon codes for a single amino acid. The genetic code is shown in the table below


    Figure 2: The Genetic Code. To find the amino acid for a particular codon, find the cell in the table for the first and second bases of the codon. Then, within that cell, find the codon with the correct third base. For example CUG codes for leucine, AAG codes for lysine, and GGG codes for glycine.

     Reading the Genetic Code

    If you find the codon AUG in the table above, you will see that it codes for the amino acid methionine. This codon is also the start codon that establishes the reading frameof the code. The reading frame is the way the bases are divided into codons. It is illustrated in the figure below. After the AUG start codon, the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They do not code for any amino acids.


    Figure 3: Reading the Genetic Code. The genetic code is read three bases at a time. Codons are the code words of the genetic code.

     Characteristics of the Genetic Code

    The genetic code has a number of important characteristics:

    • The genetic code is universal. All known living things have the same genetic code. This shows that all organisms share a common evolutionary history.
    • The genetic code is unambiguous. This means that each codon codes for just one amino acid (or start or stop). This is necessary so there is no question about which amino acid is the correct one.
    • The genetic code is redundant. This means that each amino acid is encoded by more than one codon. For example, in the table above, four codons code for the amino acid threonine. Redundancy in the code helps prevent errors in protein synthesis. If a base in a codon changes by accident, there is a good chance that it will still code for the same amino acid.

    Cracking the Code

    The double-helix structure of DNA was discovered in 1953. It took just 8 more years to crack the genetic code. The scientist who was mainly responsible for deciphering the code was American biochemist Marshall Nirenberg, who worked at the National Institutes of Health. When Nirenberg began the research in 1959, the manner in which proteins are synthesized in cells was not well understood, and messenger RNA had not yet been discovered. At that time, scientists didn't even know whether DNA or RNA was the molecule that was used as a template for protein synthesis. Nirenberg, along with a collaborator named Heinrich Matthaei, devised an ingenious experiment to determine which molecule, DNA or RNA, has this important role and also to begin deciphering the genetic code.

    Nirenberg and Matthaei added the contents of bacterial cells to each of 20 test tubes. The cell contents provided the necessary "machinery" for the synthesis of a polypeptide molecule. The researchers also added all 20 amino acids to the test tubes, with a different amino acid "tagged" by a radioactive element in each test tube. That way, if a polypeptide formed in a test tube, they would be able to tell which amino acid it contained. Then they added synthetic RNA containing just one nitrogen base to all 20 test tubes. They used the base uracil in their first experiment. They discovered that an RNA chain consisting only of uracil bases produces a polypeptide chain of the amino acid phenylalanine. This experiment showed not only that RNA (rather than DNA) is the template for proteins synthesis. It also showed that a sequence of uracil bases codes for the amino acid phenylalanine. The year was 1961, and it was a momentous occasion. When Nirenberg presented the discovery at a scientific conference later that year, he received a standing ovation. As Nirenberg puts it, "...for the next five years I became like a scientific rock star."

    After Nirenberg and Matthaei cracked the first word of the genetic code, they used similar experiments to show that each codon consists of three bases. Before long, they had discovered the codons for all 20 amino acids. In 1968, in recognition for this important achievement, Nirenberg was named a co-winner of the Nobel Prize in Physiology or Medicine.


    • The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The four bases make up the "letters" of the code. The letters are combined in groups of three to form code "words," or codons, each of which encodes for one amino acid or a start or stop signal.
    • AUG is the start codon, and it establishes the reading frame of the code. After the start codon, the next three bases are read as the second codon, the three bases after that as the third codon, and so on until a stop codon is reached.
    • The genetic code is universal, unambiguous, and redundant.
    • The genetic code was cracked in the 1960s mainly by a series of ingenious experiments carried out by Marshall Nirenberg, who won a Nobel Prize for this achievement.


    1. Describe the genetic code.
    2. Explain how the genetic code is read.
    3. Identify three important characteristics of the genetic code.
    4. Summarize how the genetic code was deciphered.
    5. Use the table entitled The Genetic Code, shown above, to answer the following questions.

      a. Is the code depicted in the table from DNA or RNA? Explain your reasoning.

      b. Which amino acid does the codon CAA code for?

      c. Does UGA code for an amino acid? Why or why not? If so, which one?

      d. Look at the codons that code for the amino acid glycine. How many of them are there? What are their similarities and differences from each other?

      e. Imagine that you are doing an experiment similar to the one performed by Nirenberg and Matthaei with 20 test tubes, each containing bacterial cell contents and all 20 amino acids, with one type of amino acid labeled in each tube. If you added synthetic RNA containing only the base cytosine, a polypeptide chain consisting of which amino acid would be produced? Explain your answer.

    6. True or False. One codon can encode for more than one amino acid.

    7. True or False. The codons for tyrosine in plants are the same as ones that encode for tyrosine humans.

    8. True or False. The start codon encodes for an amino acid, in addition to its function establishing where the reading frame starts.

    9. How many possible codons are there?

      A. 64

      B. 20

      C. 3

      D. It depends on the species

    10. How many common amino acids are there in proteins?

      A. 64

      B. 20

      C. 3

      D. 4

    Explore More

    Comparing DNA sequences is vital to understanding evolutionary relationships between organisms. Check out more here: 

    Marshall Nirenberg is known in the world of Genetics for cracking the genetic code, learn more about him here:  


    Image Attributions

    [Figure 1] 
    Credit: By Bas E. Dutilh, Rasa Jurgelenaite, Radek Szklarczyk, Sacha A.F.T. van Hijum, Harry R. Harhangi, Markus Schmid, Bart de Wild, Kees-Jan Françoijs, Hendrik G. Stunnenberg, Marc Strous, Mike S.M. Jetten, Huub J.M. Op den Camp and Martijn A. Huynen (doi: 10.1093/bioinformatics/btr316) [CC BY 2.5 (], via Wikimedia Commons; 
    License: CC BY-NC 3.0

    [Figure 2] 
    Credit: By NIH [Public domain], via Wikimedia Commons; 
    License: CC BY-NC 3.0

    [Figure 3] 
    Credit: By Madprime (Own work) [CC0, GFDL (, CC-BY-SA-3.0 ( or CC BY-SA 2.5-2.0-1.0 (], via Wikimedia Commons; 
    License: CC BY-NC 3.0